1. Field of the Invention
The present disclosure generally relates to a method and system for estimating the relative frequency offset between a Base Station Transmitter and Mobile Station Receiver for a wireless communications system, and in particular but not exclusively, for a Wideband Code Division Multiple Access (“W-CDMA”) radio telecommunications system.
2. Description of the Related Art
In a radio communications system, signals are typically transmitted from the fixed location of a Base Station (“BS”) (such as a radio transmission tower) and travel towards a moveable Mobile Station Receiver (“MSR”) (such as a mobile phone) via many different physical paths. The signal received by the MSR will rarely be in exactly the same form as the original transmitted signal, due to influences from the physical environment during propagation. Along each propagation path, the signal may be subjected to interference from background noise, reflection off objects in the physical environment, signal attenuation due to the distance of the path between the BS and the MSR, or be subjected to frequency variations due to the relative motion between the MSR and the BS (i.e., resulting from the Doppler Effect). By the time the transmitted signal is received by the MSR, the signals from each of the different propagation paths may each be attributed with a different phase delay.
In order to improve the reception capability of the MSR, it is desirable to receive signals from different propagation paths and combine them in a coherent manner. For example, a transmitted signal which is received by the MSR via two different propagation paths should be combined in phase to achieve the best reception results. If those two signals were received and combined by the MSR in anti-phase (or 180° in phase apart) then those two signals will cancel each other out, which thus leads to loss of information or performance. Therefore, it is necessary to estimate the phase of each received signal from each path and correct any phase differences between those received signals before combining the individual paths.
Some systems (such as 3GPP UTRA-FDD systems) use a pilot channel, which is a continuous modulation of a known bit sequence in the transmitted signal. Using the pilot channel, it is possible to estimate the phase of different signals received corresponding to different propagation paths before combining them together. As a by-product to measuring the phase of pilot symbols received by a coherent MSR receiver, it may also be possible to estimate the relative frequency offset that exists between the BS and MSR.
The clocks in the BS and MSR are not synchronized, thus creating a frequency offset between the generated frequencies in the BS and MSR. It is desirable for the MSR to estimate, and compensate for, this frequency offset since without such correction, degradation of the signal will result and thus affect the MSR's reception performance. One technique used to estimate the frequency offset involves applying an inverse tangent function on the demodulated pilot symbols and its complex conjugate. This technique requires division, which is complex to implement. Also, the output of the inverse tangent function can only be calculated to approximation (by using Taylor series or lookup tables) which inevitably results in errors or imprecision.
It is desired to provide an improved processing method and system for estimating the frequency offset between the BS and MSR, or to at least provide a useful alternative to the prior art.